1. Field of the Invention
The invention relates to molecular fluorine lasers, and particularly to precise wavelength control and monitoring of a VUV beam.
2. Discussion of the Related Art
In the near future, production of integrated circuits for computer chips is expected to utilize microlithography based on a molecular fluorine (F2) laser operating at a wavelength of 157 nm. According to the International Technology Roadmap for Semiconductors, Semiconductor Industry Association, 1999 Edition (International SEMATECH, Austin, Tex. 1999, which is hereby incorporated by reference, the 157-nm laser will permit production of integrated circuits for computer chips with critical dimensions of 100 nm and perhaps as low as 50 nm. Currently, all basic technologies for 157-nm lithography are under thorough investigation and intense development (see J. A. McClay and A. S. L. McInture, xe2x80x9c157 nm Optical Lithography: The Accomplishments and the Challenges,xe2x80x9d Solid States Technol. 42, 57-68 (1999), which is hereby incorporated by reference). For the accurate design of projection optics for lithography tools, precise knowledge of the spectral properties of F2 lasers designed for lithography, particularly the emission wavelength and bandwidth, is desired.
Lasing in molecular fluorine was first reported by Rice et al. in 1977 (see xe2x80x9cVUV Emissions From Mixtures of F2 and the Noble Gasesxe2x80x94a Molecular F2 Laser at 1575 Angstromsxe2x80x9d, Appl. Phys. Lett. 31, 31-33 (1977), which is hereby incorporated by reference). Since then, the 157-nm F2 laser has undergone significant development. Today, F2 lasers operating at 157 nm are available for both low and high-power applications. Some details of the output characteristics of the F2 laser have been given by Kakehata et al (see Output Characteristics of a Discharge-Pumped F2 Laser (157 nm) with an Injection-Seeded Unstable Resonator,xe2x80x9d J. Appl. Phys. 74, 2241-2246 (1983), which is hereby incorporated by reference). A summary of the development and properties of F2 lasers operating at 157 nm is given in the review paper of Hooker and Webb (see S. M. Hooker and C. E. Webb, xe2x80x9cProgress in Vacuum Ultraviolet Lasers,xe2x80x9d Prog. Quantum Electron. 18, 227-274 (1994).
Because the index of refraction of optically transmissive materials such as CaF2 and BaF2 that may be preferred for use with the 157-nm illumination and projection optics varies rapidly with wavelength, it is desired that the wavelength of the laser be known to high accuracy. An accuracy of approximately one part in 106 is particularly desired. The first wavelengths for the F2 laser were given by Woodworth and Rice (see xe2x80x9cAn Efficient, High-Power F2 Laser Near 157 nm,xe2x80x9d J. Chem. Phys. 69, 2500-2504 (1978), which is hereby incorporated by reference). By using an evacuated Seya-Namioka spectrometer to record a spectrum on Kodak short-wave radiation (SWR) film, they measured wavelengths of 156.71(1) nm, 157.48(1) nm, and 157.59(1) nm for the three observed lasing transitions, the main line being that at 157.59 nm (see M. J. Weber, Handbook of Laser Wavelengths (CRC Press, Boca Raton, Fla., 1999), which is hereby incorporated by reference, and in earlier versions of this table as being wavelengths in air. However, these values appear to represent wavelengths in vacuum).
McKee used a long-focal-length concave-grating spectrograph at the National Research Council of Canada to make measurements for two of the three lines (see T. J. McKee, xe2x80x9cSpectral-narrowing techniques for excimer laser oscillators,xe2x80x9d Can. J. Phys. 63, 214-219 (1985), which is hereby incorporated by reference. Although no accuracy or details of the measurements were given by McKee, review of the research log sheets at the National Research Council shows that the measurements were made by photographing light from the F2 laser in seventh order on a 10.7-m normal-incidence vacuum spectrograph. Wavelengths were calibrated by lines in overlapping orders from an iron hollow-cathode lamp. Light from the hollow cathode was directed to the spectrometer by a mirror mounted at 45xc2x0 to the optic axis of the spectrometer. K. P. Huber, Steacie Institute for Molecular Science, National Research Council of Canada, Ottowa, Ontario (personal communication, March 2000)). McKee obtained values of 157.5233 and 157.6299 nm, which are approximately 0.04 nm longer than those of Woodworth and Rice. These wavelengths were reproduced subsequently in the paper of Ishchenko et al. (see xe2x80x9cHighpower efficient vacuum ultraviolet F2 laser excited by an electric discharge,xe2x80x9d Sov. J. Quantum Electron, 16, 707-709 (1986), which is hereby incorporated by reference).
In view of the above, a molecular fluorine laser system is provided including a discharge chamber filled with a gas mixture including molecular fluorine and a discharge chamber buffer gas, multiple electrodes within the discharge chamber connected to a discharge circuit for energizing the gas mixture, a resonator for generating a narrow band output beam around 157 nm of a known wavelength, a wavelength selection and tuning unit for tuning the wavelength of the narrow band output beam, preferably by adjusting the gas mixture pressure, a wavelength calibration module permitting the wavelength of the narrow band output beam to be calibrated to a specific absolute wavelength. The module contains a species having an optical transition within the emission spectrum of said molecular fluorine laser system either for optically interacting with a beam portion in a region around 157-nm or for emitting light to be coincident at a spectrograph with the beam portion. The laser system is configured either to measure effects of interaction of the species with the beam portion as the wavelength of the narrow band output beam is scanned or to compare relative positions of the beam portion and lamp light after the spectrograph. The system further includes a processor for calibrating the wavelength of the narrow band output beam based wither on the measured effects of the interaction of the species with the beam portion or the comparison of the beam portion and lamp light.
An ArF-excimer laser system may include a gas mixture including molecular fluorine and a discharge chamber buffer gas, wherein argon may be added as an active rare gas for an argon fluoride laser gas mixture. The wavelength calibration module permitting the wavelength of the narrow band output beam to be calibrated to a specific absolute wavelength preferably includes platinum which has an optical transition within the emission spectrum of the excimer laser, e.g., ArF, or molecular fluorine laser system.
In preferred embodiments, a calibration module buffer gas is also contained within the wavelength calibration module. The species having the optical transition around 157 nm preferably includes platinum, and the buffer gas preferably includes neon.
The system may further include a photodetector for measuring an intensity of the output beam after the output beam traverses a volume of the module including the optically interacting species. The photodetector may measure a phototabsorption by the species of the beam portion.
The laser system may include a galvanometer for measuring a potential difference between two points separated by a volume of material including the optically interacting species filling the module. The module may include a galvatron, wherein a current is flowed between an anode and a cathode of said galvatron to cause material of the cathode to fill the galvatron in gaseous form, and the optogalvanic effect may be used for the wavelength calibration.
The system preferably further includes a sealed enclosure connected to the resonator and providing an output beam path for the beam as it exits the resonator that is substantially free of VUV photoabsorbing species so that the energy of the beam can reach an application process without substantial attenuation due to the presence of photoabsorbing species along the output beam path. A beam splitter within the enclosure may erve to separate part of the beam to be incident at the wavelength calibration module. The part of the beam that is separated at the beam splitter to be incident at the wavelength calibration module may be directed along a second beam path after the beam splitter within the enclosure that is protected from being substantially attenuated by said VUV photoabsorbing species. In operation of the molecular fluorine laser system, the wavelength calibration module calibrates the wavelength of the output beam by detecting substantially the part of the beam that is directed to the wavelength calibration module from the beam splitter module along the second beam path within the enclosure.
According to this embodiment, the system may also include a second beam splitter for reflecting another portion of the beam along a third beam path leading to a relative wavelength measurement module which is calibrated to the absolute reference of the wavelength calibration module. The same or a difference processor may receive a signal from the relative wavelength measurement module for monitoring the wavelength of the output beam in a feedback arrangement for controlling the wavelength of the output beam. The relative wavelength calibration module may include a monitor etalon and an array detector.
A window made of one of the substantially VUV transparent materials may separate an atmosphere within the sealed enclosure from that within the wavelength calibration module. A spectrograph may be coupled to the enclosure which receives a reflection of the beam portion that is incident at the window for monitoring the wavelength of the output beam.
The gas mixture of the molecular fluorine laser may include two or more buffer gases. A gas handling unit of the laser system may receive instruction signals from the processor for adjusting the amount of one or more of the buffer gases based on the instruction signals to control one or more spectral parameters of the laser pulses, such as wavelength and/or bandwidth. For example, first and second buffer gases, such as neon and helium, may be correspondingly adjusted so that the wavelength is adjusted to a desired value by adjusting the relative concentrations of the first and second buffer gases while the total pressure of the gas mixture is maintained substantially the same. According to another example, the first and second buffer gases may be correspondingly adjusted so that the bandwidth is adjusted to a desired value by adjusting the total pressure of the gas mixture while the relative concentrations of the first and second buffer gases is maintained substantially the same. In a third example, both the total pressure and relative concentrations of the buffer gases may be adjusted for adjusting both the bandwidth and the wavelength to desired values.